1. Field of the Invention
The present invention relates generally to electrical switching apparatus and, more particularly, to a bypass transfer switch including an interlock system. The invention also relates to an interlock system for a bypass transfer switch.
2. Background Information
Alternate power sources are provided for any number of applications which cannot withstand a lengthy interruption in electric power. Typically, electric power is provided from a primary source with back-up power provided by a secondary source. Often, the primary source is a utility power source and the secondary source is an auxiliary power source, such as an engine driven generator or a second utility source. The transfers between the two power sources can be made automatically or manually.
Transfer switches are well known in the art. See, for example, U.S. Pat. Nos. 5,397,868; 5,210,685; 4,894,796; and 4,747,061. Transfer switches operate, for example, to transfer a power consuming load from a circuit with a normal power supply to a circuit with an auxiliary power supply. Applications for transfer switches include stand-by applications, among others, in which the auxiliary power supply stands-by if the normal power supply should fail. Facilities having a critical requirement for continuous electric power, such as hospitals, certain plant processes, computer installations, and the like, have a standby power source, often a diesel generator. A transfer switch controls electrical connection of the utility lines and the diesel generator to the facility load buses. In many installations, the transfer switch automatically starts the standby generator and connects it to the load bus upon loss of utility power, and reconnects the utility power to the load bus if utility power is reestablished.
In the case of a generator driven auxiliary power source, power must be stabilized before the transfer can be made to the secondary source. In any event, the two power sources cannot be connected to the load simultaneously unless they suitably match their respective voltages, frequencies and phases. Some transfer switches affect an open transition between the power sources, that is, one is disconnected from the load bus before the other one is connected. Other transfer switches provide a closed transition wherein the one source is connected to the load bus before the other source is disconnected, in order that both power sources are connected in parallel during the transition.
Transfer switches commonly used to connect alternate power sources to a load, including networks, utilize a pair of switches each connecting one of the sources to the load. In order to prevent connecting unsynchronized sources together, the operation of the two switches is coordinated, typically by a mechanical interlock, in order that only one switch at a time can be turned on. Each transfer switch generally comprises a pair of circuit interrupters combined with a drive input and a linkage system. The preferred types of circuit interrupters have been molded-case switches and molded-case circuit breakers because these types are commercially available in a wide array of sizes and are relatively economical compared to other options. The circuit breaker enclosure or cassette of some of these circuit interrupters have a dual lever interlock feature. The preferred type of drive input depends on the application for the transfer switch. Motors are often preferred, but at other times there is a clear preference for manually-operated mechanisms.
One type of breaker bypass system is a four-way or four-breaker bypass system which typically comprises two adjacent normal line breakers positioned on top of two adjacent emergency line breakers. Such a system must comply with the logic scheme wherein if one breaker is on while two selected breakers are held off, the last breaker may be either on or off. Known four-way bypass transfer switches have accomplished this logic scheme through use of two-way cable interlocks between normal and emergency breakers for the transfer switch and bypass sections. However, this approach requires additional interlocking when performing bypassing operations, in order to ensure that unsynchronized paralleling of normal and emergency sources does not occur. Specifically, a plurality of commercially available locking assemblies such as, for example, KIRK® keys, are required between the normal breaker automatic transfer switch (ATS) and the emergency breaker (bypass) as well as between the emergency breaker (ATS) and the normal breaker (bypass). A KIRK® key is a well known keyed locking device. Utilizing KIRK® keys enables the four-way breaker interlocking scheme to be achieved, but it adds complexity and time to the bypass operation. In critical power applications, such as those previously discussed (e.g., without limitation, hospitals) where bypassing must occur very rapidly in order to resist an interruption in power, the added time and complexity of the KIRK® key bypass interlock scheme is unacceptable.
There is a need, therefore, for a simplified four-way bypass interlock system.
There is, therefore, room for improvement in bypass transfer switches and in interlock systems therefor.